In a bonded structure consisting of a base material and an adhesive sheet to be bonded to the base material, a primer layer is often provided between the base material and the adhesive sheet.
The primer layer is usually formed by coating a primer composition comprising a polymer (primer polymer). Two types of primer layers have conventionally been used. One is a cured primer layer formed by curing a coated primer composition and the other is an uncured primer layer that can be obtained by simply coating and drying a primer composition.
For example, the cured primer layer is disclosed in Japanese Patent Application Laid-open No. 7-52305. The Patent Application discloses an exterior decorative material for buildings, which is a bonded structure formed from a base material made from a mixture of an inorganic material (e.g., a calcium silicate material) and a fibrous material, a primer layer containing a cured resin formed on the surface of the base material, and a decorative sheet adhered to the surface of the primer layer. Typically, the decorative sheet has an adhesive layer that adheres to an exterior member via the adhesive layer. The cured resin is formed from a curable resin. Examples of curable resins that form the cured resin include an epoxy resin, polyisocyanate resin, urethane resin, and acrylic resin.
Such a cured primer layer is advantageous for increasing weather resistance, water resistance, plasticizer resistance, and the like. “Plasticizer resistance” means a plasticizer contained in a base material is prevented from moving to the adhesive layer of an adhesive sheet. If the plasticizer moves to the adhesive layer, the cohesive force of the adhesive layer is impaired. A decrease in the cohesive force induces a decrease in the adhesive force of the adhesive sheet with the base material (the primer layer) and may cause the adhesive sheet to release from the bonded structure during use. As an example, a base material has an outermost layer consisting of a plasticized resin layer that contains a resin and a plasticizer for the resin. Such a base material is used as an interior decorative board. The adhesive sheet (e.g. a decorative sheet) is used for repairing the interior decorative board, remodeling the inside and outside walls of buildings formed from the interior decorative board, and the like by adhering to the surface of the interior decorative board.
On the other hand, the uncured primer layer is disclosed in Japanese Patent Application Laid-open No. 2001-199195. The primer composition disclosed in the Patent Application contains an acrylic resin (a core-shell type acrylic emulsion resin) as a primer polymer.
In the cured primer, after applying the primer composition containing a curable resin to the surface of a base material, the primer layer must be aged for curing. A comparatively long time is required for completing the aging of the primer layer. In addition, in the case of a two-liquid type primer composition consisting of a main agent liquid and a curing agent liquid, these two liquids must be weighed and blended in a prescribed proportion at the production site. A one-liquid cured primer composition (moisture cured, for example) can be processed more simply than the two-liquid type. However, this type of primer is inconvenient due to difficulty in storing for a long period of time.
Some primer layers may be used without curing such as in the case when an acrylic resin is used (described above). However, such acrylic resin primer layer exhibits comparatively high affinity to plasticizers. Therefore, it is difficult to prevent a plasticizer from moving from the base material to the adhesive layer of an adhesive sheet when an acrylic primer is used without curing.
Alternatively, it is possible to use polyurethane as a primer polymer without curing the primer layer. However, such a primer layer has other types of problems associated with such a primer layer. Specifically, since polyester polyurethane is a fairly common polyurethane and is easily hydrolyzed, it is difficult to increase the water resistance. A decrease in the water resistance induces a decrease in the adhesive force of the adhesive sheet with the base material (the primer layer) and may cause the adhesive sheet to release from the bonded structure during use.
A primer layer formed from an aqueous primer containing a primer polymer dispersed in an aqueous medium particularly tends to decrease the water resistance. Increased water resistance is required for adhesive sheets used as an internal decorative sheet on the walls or floors of rooms where water is used such as a bathroom, kitchen, lavatory, and indoor pool.